It would seem to me that you can't add these numbers directly. The resulting number we get from the meter is like a DC value on a meter, but it is derived from a chaotic waveform composed of noise from every connection that exists while you are measuring. I think you would have to add the squares and take the root in order to get a good guesstimate.
Nothing in electronics is that easy as a scalar addition of two entirely different wave forms unless the delta T approaches 0.
That's the point. Measurement uncertainty.
If you used a spec-an, you could read what is a harmonic and what isn't. That might help the confidence aspect.
Couldn't you just convert into percentages and add those for a total?
Doing just that with your given numbers (-110dB + -105dB) gives me a total of 0.0008785% or -101.1251dB.
No idea if that is anywhere near reality though. Someone else would have to disprove or confirm.
Not sure if the limited number of decimal points of the online calculator I used is sufficiently accurate though.
If I treat the numbers as dBV, convert to Vrms, compute the difference and then back to dB I get -106.5dB with a generator that has -110dBV and a result of -105dBV. Not sure that's correct though.
Unless you know the phase of each component you have to do an RMS calculation as a best guess. Only if you know that the sig gen and circuit generate a given harmonic at the same phase can you do a straight arithmetic difference.
Hmmm. Interesting approach, although it would be a bit different for my numbers because -105 is the result of a generator at -110 and ´X´ resulting in finally -105. So in your approach X would be -110 - -105. I think.